Method of fractionation



Jan. 4, 1938. P. J. RoELFsEMA 2,104,310

METHOD OF FRACTIONATION Filed July 14, 19s4` 3 sheets-shewb 1 Jan. 4, 1938. P. J. RoELFsEMA 2,104,310

METHOD OF FRACTIONATION Filed July 14, 1954 5 Sheets-Sheet 2 E otcou wmos Jan. 4, 1938. `F. J. RoELFsEMA METHOD oF FRAGTIONATION Filed July 14, 1934 3 Sheets-Sheet 3 Lvmcouccu N.

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lnvenor; Pcirus Roelfsema Patented .1.11.4,1938

UNITED STATES PATENT OFFICE ammo 4' METHOD OF FRACTIONATION Petrus Jurjen Roelfsema, Walnut Creek, Calif., signor to Shell Development `(Jompany, San Francisco, Calif., a corporation of Delaware Application July 14, 1934, Serial No. 735,216

2 Claims.. (Cl. 196-72) This invention pertains to methods' of fractionation and is more particularly concerned with means of reducing, separately or simultaneously, the fluctuations in 'the properties of the product, the load on the fractionating ap-f paratus, and/or increasing the yield of ,desired products.

In the ordinary practice of fractionating petroleum distillates,-the composition of the feed unavoidably fiuctuates within certain often very wide limits. These fluctuations in the feed composition may cause corresponding changes in the load on the apparatus and in the properties and yields of the product. Under such conditions, a fractionating column will operate at a load which will continuously vary, operations being carried out at a load higher or lower than the optimum desired. Y

If the percentage of volatile components pres- .20 ent in the feed decreases and the reflux remains constant, the load on the column will decrease to one lower than the optimum. At the same time the reflux ratio will exceed the one corresponding to a desired and economical yield, so that the process becomes wasteful of heat.

If the percentage of volatile components in the feed increases and the reflux remains constant the load on the column will exceed the optimum and at the same time the reflux ratio will be 0 lowered and the process will then fall to produce the desired economical yield.

Numerous methods of control and regulation have been devised to preclude the occurrence of such undesirable conditions in the operation of a fractionating system, these methods being directed to correlating the conditions of temperature, pressure, heat input, etc., with the desired compositions and rate of flow of the material passing to, through and from the system.

40 It is, however, believed that the prior art has failed to make a proper distinction between th following three main variables:

(1) Properties of product, such as the final boiling point, the dew point, the initial boiling point, ete.

(2) Yield of product as determined by the reflux ratio,

(3) Load on the column as determined by th amounts of reflux and top product. 50 'Ihese variables were usually regarded as essentially interdependent, and a good control of one of them was achieved only at the sacrifice of another. Y

The three-fold object of this invention is,

55 therefore, to control the specified properties of the final product, the amounts of yield, and the load on the apparatus, as factors largely, if notentirely, independent from each other.

First, the specified properties of the desired product are Vmaintained constant by regulating 5 the heat input for a fixed working pressure as a function of the controlled significant temperatureof the system.

Second, the yield of the product as expressed in v% of the feed is maintained constant and 10 preferably at its optimum value by maintaining a constant ratio between the amount` of reflux returned to the column and the amount of top product withdrawn from the system. The control of the yield is assured in this way as long 15 amount of overhead, i. e. reflux plus top product.

Since the load on the column isdetermined by the sum of reflux and top products in mois, provided, as is usually the case, that the down-spout capacity is ample, the control of this sumwill '25 control the load. No direct control of the sum of mols of reflux plus top product being always practicable, it was discovered that an almost ideal approach is made by controlling the sum of the gallons of reux plus gallons of top product.v

My invention will be fully understood from the y following description taken with reference the drawings of Figs. 1, 2, and 3, which represen diagrammatically` and partly in cross-section, three preferred forms of apparatus for practicing the process.

Referring to Fig. 1, the apparatus consists of a fractionating column I, provided with suitable rectifying means, such as plates orv packing, not shown; the column is also provided with a pipe connection 2 anda manifold 3 for introducing the feed, after passing it, if desired. through the heatv exchangers 25, or any other form of heating apparatus.'V Steam or any other suitable heating 45 agent may be delivered to4 the fractionating column in any conventional way, such, for example, as by means of a reboiler B as shown on. the drawings. The rate of flow of the'heating medium is regulated as a function of the temperature of the -vapor near the top of the column by means of controller l1, or as a function of the final boiling point of the condensed vapor phase by means of controller I9, either of which may be adapted to regulate valve 2l. Liquid 55 valve 22, actuated by controller I8, which is re` and vapor withdrawal pipes l and l are provided respectively near the bottom and top of the column. The withdrawal of the bottom product through pipe 4 is regulated by a valve 2i, responsiveto the liquid level controller 28. The vapor pipe I passes through a.l partial or total condenser 8, having means I for circulating a cooling medium automatically controlled by a sponsive to the pressure in column I or tank 8. As an alternative, the same eifect may be obtained by means of a by-pass pipe 29 controlled by a valve 30 or by a heating coil l2 in the surge tank 8. The condenser is connected lwith a surge tank 8, which is provided near its top with a vapor pipe 8 I, through which, lf desired, final product may be'withdrawn in vapor form, in case that only a partial condensation is effected. Tank 8 is also provided with a liquid level controlling means III. This means Il is adapted alltomatically to actuate the valve Il on the feed line 2 in such a manner as to restrict the feed flow whenever the liquid level in the tank l rises, or correspondingly to increase said ilow when the liquid level in tank l falls.

As an alternative, instead of controlling the feed as a function of the rate of flow .of condensate to the surge' tank 8 indirectly, that is, through the. intermediary of the liquid level controller I8, the rate of feed may be controlled directly as a function of the rate of flow. of the condensate by installing an oriilce meter IIIA between the condenser and surge tank 8 and connecting it in a way identical to that of controller I8. This liquid flow meter willin this case perform all the functions of the liquid level controller I8, which may accordingly be. dispensed with.

A pump I2 is provided to withdraw the condensed distillate at a constant rate from the tank 8 through pipe 9. A reflux pipe I5 connects the pump I2 with the top of the column I and a pipe I 4 is used to withdraw the top product from the system in case that total condensation is effected. Flow ratio control valves ISa and I8b regulate the ilow in pipes Il and Il respectively.

In cases where column I has suilicient capacity to handle considerable surges, and feed regulation by means of controller Il and valve II is not necessary, valves Ila and I8b may be' For' regulating the desired properties of the top product, a temperature control means I1 or a final boiling temperature controller I8, such, for example, as disclosed in J. I". M. Taylors Patent 1,876,364, may be used to regulate the' flow of heating medium through a valve 28 into the reboiler I8. If a fully automatic control and regulation such as described. is not desired, the

same results may be approached vby wording either the dewpoint temperature or the tlnal boiling point and by manually regulating the flow of the heat medium to the reboiler Vin accordance with the recorded temperatures of the'overhead or the top product;

1 Fig. 2 represents another embodimentA of my invention particularly suited-when the composition of the bottom product is to be controlled.

In this case, a temperature control means ITB may be used to maintain a predetermined temperature at the bottom oi' the column in accordance with the working pressure and with the deheated, partially or entirely vaporized, for example, by application of direct heat or. by passing through one or several heat exchangers 2l. Vapors within the column pass countercurrently to and in contact and heat exchange relationship with the descending liquid. The ultimate vapor mixture at the top of the column is withdrawn by means of pipe I, condensed in the con denser 8, and is collected in the surge tank 8 from which it is continuously withdrawn by pump I2 and divided in a constant ratio by valves -IIa and lib between the -reilux pipe Ii and the liquid withdrawal pipe I4.

The liquid product lformed within the column I is collected at the bottom of the column and may be withdrawn through pipe l at a rate regulated by valve -2| responsive to levelcontroller 2l, after being passed through the heat exchanger 2l, or by-passed by means of pipe 8i.

A substantially constant pressure is automatically maintained in thesystem by the controller It, which actuates valve 22, controlling the iiow Y of 'cooling medium in pipe 1, or valve 8l, controlling the ilow of vapors by-passed around the condenser by pipe 28, or again by regulating the steam owing through a coil 82 which may be installed for the purpose in the surge tank 8. Controller I 8 may be made responsive to the pressure variations in column l, or, if desired, in tank 8.

supposing that the fractionating unit of Fig. 1 is operated for a specification top product, and the operation is to be maintained for an optimum yield, and assuming that the fractionating process has reached the point oi' stable operation, a certain rate of feed is then maintained-through the valve` II, corresponding to a certain liquid level in tank 8. Ii'- now the composition of lthe feed changes so thatvthe content o! volatile components increases, an increased amount of material will reach the reflux condenser I. The amount 'of cooling medium going to thu redux condenser remaining constant,-.this apparatus will fail. to condense'all thevapors and, a's a first result, the pressure on lthe system will show ak tendency to rise. As a consequence, the pressure controller I8 will open the regulating valve 22 and thus-admit an increased amountV of cooling medium to the reflux condenser until such an increase has been realised that the former pressureis reestablished. As the increased amount o! vapors will now again be totally condensed in the redux condenser, the amount of liquid iiowing into .tank 8 will increase and'sinee a constant amount is removed from this tank by means of pump' Il, the liqiildlevelin tank lwill rise. Thlsin turn will cause the liquid-level controller I8 to act upon the feed regulating valve II in a manner which will reduce the rate of feed to the column. In 75 action of the increased head on the master cong this manner, the controlled liquid level in tank I, which depends upon the total amount of overhead. i. e. the sum of the reflux and of the top product, is used for regulating the rate of feed and therefore Vthe load on the column. The yield may' be simultaneously controlled through the valves Ila and IIb, keeping a constant ratio between the reflux and the top product withdrawn.

In this manner it is possible to obtain: (a) A constancy of overhead (reflux plus top product) which assures a continuous operation at any desired constant load, including the opti- (b) A constancy in the ratio of reflux to top product which in turn assures a constancy in the degree of fractionation;

(c) A constancy in the amount of the desired top product e (d) A cgntinuous regulation of the rate of feed in accordance with both the desired load and the desired yields; u

(e) A constancy of working pressure which, in conjunction with the temperature control I1 or I8, assures a constancy of the Specified properties of the top product by means of valve 28 regulating the heat input to the column.

A similar combination of either automatic or manual control and regulation would adjust the rate of feed in case the content of volatile components should diminish. The receding level in tank 8 would then cause the opening of the valve II to admitmore fluid, while all other Afeatures of controlv and regulation would proceed in a manner insuring a stable operation of the-system. When the fractionating unit is operated for a bottom product of specied properties, a somewhat diiferent arrangement of controlling and regulating means may be used, such, for example, as illustrated in Fig. 2. Instead of means I1 and/or I9, the temperature control means I1B may be used to control the specined properties of the bottom product in cooperation with the constant pressure control means I8.

Tank 8 may in this case be replaced by a standpipe 8B, which has a solid partition 26 separatingit into two liquid-tight compartments, communicatingfby means of a pipe 21., in which a constant flow is maintained, for instance, by a master-controller or liquid head controller, NB

actuating valve 28. The same pressure is maintained in both compartments by means of a pipe IZB connecting their vapor spaces. The purpose of this arrangement is to use the variable liquid head in the upper section of tank 8B to actuate the master-controller MB, which in turn reguv lates the flowof the reux by means of valve 2l,

the ow of the feed in pipe 2 by means of valve Il, andr the flow of the top product in pipe 21 by means of valve 28. The master controller is connected to both the liquid and the vapor phases in the standpipe 8B, so that it is not influenced by variations of the total pressure in the system but only by variations in the liquid head.

Assuming then, as in the rst case, that the amount of volatile components in the feed to column I increases, the pressure of the system will rise, and the pressure controller I8 will admit more cooling medium to the condenser 6, thus reestablishing the pressure but increasing the amount of condensate. The liquid head in the upper section of standpipe 8B will accordingly rise, nominally tending to increase both the flow of the top product in pipe 21 and the flow of the reflux in pipe ISB. However, the

troller or liquid head' controller IIIB is such that the latter will reduce the opening in both valve 28 and valve 24, maintaining both ows constant at the desiredl rates. Simultaneously the master controller will reduce the rate of admission of feed tocolumn I by partially closing the valve in standpipe 8B and bringing the whole system back to normal'operation. An opposite automatic adjustment will take place if the amount of the lighter components in the feed decreases since a fall of the level in standpipe 8 will in that case actuate the master controller to open valves 24, 28 and Il wider, thus keeping the flow of the reflux and of the top product constant and increasing the rate of feed admission.

It is obvious -although I have described the use of both a liquid level controller .and a static head controller as illustrated by Figs. 1 and 2 to regulate the load upon the column or the amount of yield, these apparatus may be used interchangeably whether the system is operated for a top or a bottom product. As an example, a system comprising a standpipe 8B, but 0perated for a top product is shown in Fig. 3. The same result might also achieved by means of registration and manual control at these various points.

While in the two examples given the top product is removed as liquid, it will be realized that the method applies equally well if it should prove desirable to remove it in vapor form.

Again, while the method of feed regulation has been used simultaneously with the control of the temperature regulating the heat input into the reboiler I8, and with control of the reflux ratio, it should be understood that these items are dis- II, thus ultimately decreasing the liquid head tinctly separate and may be used 'individually one without the other.

Moreover, this system of control and regulation is adapted not only to operations at atmospheric and higher pressures, but may readily be used at any pressure including any degree of vacuum.

It is obvious, that these principles of control may also be applied to a whole system comprising several fractionating units in series and/or in parallel. f

I claim as my invention:

1. In-a process of fractional distillation, the steps of counterflowing in a fractionatlng zone a vaporized hydrocarbon mixture and a reflux formed in a condensing zone, automatically maintaining a constant pressure in the fractionating zone by regulating the rate of indirect cooling in the condensing zone solely as a function oi.' the pressure in the fractionating zone, automatically regulating the flow of the material being fractionated to the fractionating zone as an inverse function of the rate of flow of the condensate formed in the condensing zone, automatically maintaining a constant reflux ratio, and automatically regulating the heat input to the fractionating zone as an inverse function of the temperature at the top of said fractionating zone.

2. In a continuous process of fractional disi tillation wherein a mixture of several components is separated into vaporized top and liquid bottom products, the steps of condensing the vaporized top product substantially under the pressure of the distillation zone, accumulating the condensate in a pool of restricted cross section and substantial depth, returning one portion of the condensate pool by regulating the rate of condensate transfer to the accumulating mue, maintaining the rate of reflux constant irrespective of the variations in the depthot the pool,` regulating the rate oi' feedto the distillation' zone inversely to variations in the depth oi the condensate pool, and automatically regulating the heat input to the fractionating zone`as an inverse function of the temperature of said tractionating zone.

PETRUS J URJEN ROELFSEMA. 

